1. Field of the Invention
This invention relates to a printing head apparatus for a dot printer, and more particularly to a printing wire unit efficient in responsiveness at the time of high-speed printing.
2. Description of the Prior Art
Dot printers which can print characters, numerals and other symbols on printing paper with a plurality of dots grouped together are known and have been widely used as printers for the computer data processing systems which have been developed in recent years because they can provide a character of any desired configuration by a certain combination of dots.
In order to increase the printing speed of such a dot printer, it is required that a printing head carrying printing wires moves at a higher speed in the direction of a line or a column in relation to a platen or printing paper. However, since the speed at which the head moves is naturally limited by the responsiveness of the electromagnetic actuator which drives the printing wire and other factors, in the prior art, in order to enable high-speed printing, a plurality of printing wires are arranged in alignment in the longitudinal direction of the platen at regular intervals and a group of printing wires are driven simultaneously in accordance with the respective printing commands. This kind of printer is known as a dot line printer.
The structure of a conventional printing unit including such an electromagnetic actuator and a printing wire driven thereby is shown in FIG. 13. The unit is provided with a printing spring c having the configuration of a thin rectangular sheet with an armature a disposed at an intermediate position and a printing wire b carried at one end; and the core e of an electromagnetic actuator d which is opposed to the armature a; the other end of the printing spring c being secured to a base portion f.
When the electromagnetic actuator d is energized in the above-described structure, the armature a is attracted by the core e, whereby the printing spring c is caused to bend such as to drive the printing wire b toward the platen and in the direction indicated by the arrow.
In the above-described conventional device, however, since the spring c is supported only at one end and has a rectangular configuration with a uniform section, the total weight of the spring c is large, and the time consumed in impacting the wire is long, so that the printing speed is disadvantageously slow.
Furthermore, with respect to the configuration of the section of the spring c, the rigidity of the portion of the leaf spring between the position at which the armature a is attached and the position at which the printing wire b is secured is small. As a result, the responsiveness of the printing wire b at the time of attracting and driving the armature a at a high speed is poor, and further, when the distal end of the printing wire b strikes the platen, it cannot produce an adequate degree of printing pressure.
It is preferable in such a dot line printer to arrange the printing wires in alignment with each other and in correspondence with all the dots which are required for printing one line. This arrangement, however, cannot be realized because it is impossible to retain a sufficiently wide space to allow the necessary number of electromagnetic actuators to be arranged, not to mention the viewpoint of economy.
To solve this problem, a device has been put into practical use which has a structure in which the number of printing wires arranged in line at certain intervals corresponds to a reduced number of dots selected from the total number of dots necessary for printing one line, and in which a frame wire carrying the printing wires is reciprocally driven only between adjacent frame wire positions in such a manner as to operate "false" simultaneous printing. This device is known as a shuttle printer.
A conventional shuttle type dot printer is composed of a shuttle frame on which a plurality of printing wires and electromagnetic actuators are arranged in alignment and secured thereto, as is disclosed in Japanese patent Laid-Open No. 131470/1984.
In this improved dot line printer, the permanent magnet for providing urging force to the printing wire in advance is eliminated in order to reduce the weight of the printing head, and hence the shuttle itself, and a balance weight which is required when driving the shuttle in the prior art is dispensed with.
A wire mask made of a thin sheet is provided on the surface facing the ink ribbon of the shuttle so as to separate the shuttle body from the ink ribbon. In an ordinary case, the wire mask is secured to the shuttle frame so as to prevent the distal ends of the wires, the wire guides, and the like from being damaged by coming into contact with the ribbon, or the printing wires themselves from being damaged.
On the front surface of the wire mask, a ribbon mask smaller in thickness than the wire mask is fixed so as to prevent the printing paper from being stained by separation of the ink ribbon from the printing paper.
However, in the conventional device in which a wire mask of a thin stainless sheet or the like is inserted between the shuttle frame and the platen and a ribbon mask is attached to the front surface of the wire mask in this way, it is difficult to hold a long wire mask such that it smoothly extends over the entire surface in a printer device having a shuttle frame of a comparatively long width, and in practice, the wire mask and the ink ribbon are sometimes caused to crinkle, thereby making it impossible to secure the smooth passage for the printing paper. This phenomenon further leads to irregular travel and damage of the printing paper, and thus to considerable degradation in printing quality.